Wells, through which production, for example, oil, natural gas, hydrocarbons, and the like are withdrawn from subterranean zones under the earth's surface, are formed by drilling down to the subterranean zones from a terranean surface (e.g., on land or subsea). The wells can include seals both near the terranean surface and near the subterranean zone to control the flow of the production. When the wells are being drilled, drilling fluid (i.e., “mud”) that is pumped from the terranean surface into the wellbore serves as one of the seals until the subterranean zone has been reached. Once the subterranean zone has been reached, the mud is removed and production string is lowered into a wellbore. An annulus between the production string and wellbore is thereafter sealed by a packer such that the production flows to the terranean surface through the production string.